This invention relates to a novel check valve construction, and specifically to a check valve for use in a reciprocating fluid pump. The invention is especially adaptable for use in pumping certain types of fluids, particularly those fluids which tend to break down under heavy and severe pressure gradients. An example of a type of fluid for which this undesirable characteristic is present is metallic paints, such as zinc-based or lead-based paints wherein the metallic paint component tends to form a weldment when stressed between a valve and seat under severe pressure.
Paints and other coating materials are typically applied using a spray gun system wherein the coating material is placed under a fluid pressure of up to 2,000 pounds per square inch (psi) in order to achieve proper atomization of the coating material for spraying. The device that develops this high fluid spraying pressure is typically a reciprocating pump which is mechanically connected to an air-operated drive motor. The air drive motor usually has an enlarged piston as compared with the pump piston so that pressure multiplication may be achieved in the system. For example, a 30:1 ratio pumping system has an air drive motor which accepts compressed air in the 0-100 psi pressure range, and the pump delivers pressurized coating material at a fluid pressure of 30 times the air pressure driving the motor. Coating material under this elevated pressure is delivered from the pump through suitable hoses to spray guns which are used to apply the material, and all elements of the delivery system must be designed to withstand the fluid pressures used. In particular, the interior components of the pump must be designed for a smooth and continuous operation at these elevated pressures, including the pump fluid valving elements which open and close during each stroke of the pump cycle. A typical system which is designed to operate under these conditions is a Model 205-457 pumping system, manufactured by the assignee of the present invention.
The reciprocating pump within which the present invention is adaptable for use has an internal mechanically reciprocated piston, and two internal valves. A foot valve is located generally below the piston, comprising a ball riding on a seat wherein upward piston movement creates a suction force which lifts the ball from the seat to admit coating material into an intake chamber, and downward piston movement forces the ball against the seat to prevent coating material from being expelled through the inlet. A piston valve comprises a ball and seat combination wherein upward piston movement forces the ball against seat to prevent fluid trapped above the piston, in a pumping chamber, from passing downward into the intake chamber, and during the downward piston movement the ball is lifted from its seat to admit coating material from the intake chamber into the pumping chamber above the piston. In this type of pump the pump outlet is generally placed above the piston in fluid coupling connection to the pumping chamber so that fluid may be expelled during both piston stokes. Both the foot valve and the piston valve are subjected to the same high coating material pressures which are present throughout the spraying system, and these components must be capable of continuous useful operation during the life of the pump.
When coating materials having a high solids content are pumped with this type of system, the solids in the coating material present a particular problem in maintaining a smooth and continuous operation of the pump foot valve and piston valve. For example, coating materials having a 25-70% content of powdered zinc tend to clog these valves and cause sticking so that the respective balls do not reliably lift from the valve seats. This is thought to be caused by the extremely high force gradients which are imposed upon the zinc particles in the region of the ball/seat contact area. Prior art valves have used virtually a line contact region between the ball and its seat, and zinc particles which are trapped in the contact area when the valve is closed are placed under extremely high forces, which have been noted to be sufficiently high to cause a weldment between the particles and the ball and seat. When a weldment occurs, the ball and seat become fused together to disable the pump. Other prior art devices have used a valve seat which is molded to the shape of the ball so as to provide a larger surface over which physical forces may be distributed and thereby to reduce the overall force on particles trapped between the ball and seat. An example of this prior art may be found in U.S. Pat. No. 3,395,890, wherein a molded plastic seat is formed to match the shape of the ball during the valve manufacture process. Still other prior art solutions to the problem have involved using a spherical valve seat in combination with a resilient ball member. An example of this prior art may be found in U.S. Pat. No. 3,787,149, issued Jan. 22, 1974, wherein the ball/seat contact area is spherical to distribute the forces over a wider area, and the ball is made from a resilient material so as to more readily release itself from the seat.
A disadvantage which is found in prior art devices utilizing a ball and seat spherical complementary shape is that under high fluid pressures the ball may have a tendency to become trapped in the seat because of the wide area of contact between these two components. Although the pressure forces on individual solid particles becomes reduced, a greater number of particles may become trapped in the common mating area to cause a sticking force which prevents the ready separation of the ball from the seat.